1. Field of the Invention
This invention relates to an optical glass exhibiting the anomaly of partial dispersing property in a long wave length region, i.e., the wave length region which is longer than the c line (6263 A).
2. Description of the Prior Art
The partial dispersion ration (Pxy) of a glass for light of two different spectral wavelengths x and y is generally indicated by expression (1): ##EQU1## WHICH IS DETERMINED ON THE BASIS OF THE DIFFERENCE IN THE REFRACTIVE INDEX .eta.F for the 4861 A line and the refractive index .eta.c for the 6263 A line. The symbols .eta.x and .eta.y represent the refractive indices of the light at wavelengths x and y, respectively. On the other hand, in normal dispersion glasses expression (2) gives the relationship between the partial dispersion ratio of the normal dispersion glass and the Abbe number (.nu.d): EQU .nu.D = (.eta.D - L) (.eta.F - .eta.c.) EQU Pxy = Axy + bxy.sup.. .nu.d (2)
wherein Axy and bxy are constants selected in accordance with the wave length x and y of light, respectively, Axy and bxy being determined by two glasses selected from normal glasses, .eta.d indicates the refractive index for the d line (5876 A) and .eta.F and .eta.c have the same meaning as in expression (1).
When pluralities of such normal dispersion glasses are assembled into e.g., a lens, the chromatic abberation between two lines on a spectrum, for example, between the F line and the C line, can be removed, but the extinction of the secondary spectrum renders super achromatism impossible.
On the other hand, if an anomalous dispersion glass exhibiting the anomaly of partial dispersion in a long wave length region is assembled with another anomalous dispersion glass exhibiting the anomaly of partial dispersion in a short wave length region, super achromatism can be attained over the whole visible light range.
The words "anomalous partial dispersion glass" mean a glass which exhibits a deviation value (.DELTA. Pxy) which is not equal to zero in expression (3) which represents the partial dispersion ration Pxy, wherein .DELTA. Pxy shows the deviation of the partial dispersion ratio from the straight line of that of a normal dispersion glass and "Pxy" is the (relative) partial dispersion of a normal dispersion glass: EQU Pxy = Pxy + .DELTA. Pxy (3)
The value of .DELTA. Pxy indicates the degree of anomaly of the partial dispersion. It is known that larger .DELTA. Pxy values result in favorable super achromatism.
In the case that the partial dispersion ratio in a long wave length region is indicated as EQU PcA' = .eta.c - .eta.A'/.eta.F - .eta.c
by selecting the C line of 6263 A and the A' line of 7682 A as x and y, respectively, in expression (1), the increase of the PcA' value and the decrease of the Abbe number .gamma.d are effective to increase the degree of the anomaly (.DELTA. PcA'). In this formula, PcA" is the (relative) partial dispersion wherein x equals C and y equals A', .eta.c is the refractive index wherein x equals c and .eta.A' is the refractive index wherein y equals A'.
Therefore, a conventional anomalous dispersion glass exhibiting the anomaly of partial dispersion in the long wave length region contains B.sub.2 O.sub.3 as the glass forming oxide since B.sub.2 O.sub.3 is effective to absorb light in the infrared region and increase the PcA' value, and PbO, which absorbs light in the ultraviolet region and decreases the Abbe number.